The present invention pertains to a constant amplitude envelope continuous phase modulator and more particularly to a scheme for greatly reducing the amount of memory required for a general purpose continuous phase modulator.
A demodulator arrangement for employing phase modulation is shown in U.S. Pat. No. 4,603,393, issued on July 29, 1986, to P. Laurant.
A continuous phase modulator is shown in publication "Digital Phase Modulation" by J. Anderson, T. Aulin and C. Sundberg, copyright 1986, Plenum Press, ISBN 0-306-42195-X. At pages 214 and 215 of this publication, variations of a modulation scheme are shown. Both variations of this modulation scheme include a phase state read-only-memory (ROM). For rational values of a modulation index h, the modulator produces one of a finite number of phase states during each symbol interval. The total number of phase states S is given by the formula S=2*Mexp(L-1), (or 2 times M raised to the power of L-1) where M is the number of symbols transmitted during a symbol period and L is the number of symbol periods. Such continuous phase modulators as the one shown in this publication encode each of the possible states as a binary value. Every location of the ROM is programmed to output the phase state given the current phase state and output symbol. For the case of a modulation index of 1/8 (h=1/8), 8 symbols per period (M=8) and 6 symbol periods (L=6), the phase state ROM would have to have 4,194,304 locations of 19 bits each for a total of 79,691,776 bits of total storage.
Large storage requirements prohibit use of such circuitry for satellite communications. In addition, such circuitry requires a great amount of power. Accordingly, it is an object of the present invention to provide a continuous phase modulation arrangement which minimizes the amount of memory required and thereby the amount of power also required.